1. Field of the Invention
This invention relates generally to electrical energy transfer circuits and, more particularly, to circuits for driving an inductive element with an applied voltage at a tuned frequency.
2. Description of the Related Art
It is often necessary to inductively transfer electrical power from one device to another, such as when a charging device is used for recharging a target device's batteries. Each device includes a coil, and the electrical power is inductively transferred from the coil in the charging device typically to the coil in the target device and then to the batteries. To maximize the transfer of energy to the target device's coil, and therefore maximize the rate at which the batteries will be recharged, while also minimizing the energy lost as heat, the coil circuit in the target device typically is tuned by using a capacitor to counteract the reactive impedance of the coil. The coil and capacitor of the target device form an L-C circuit whose impedance will be minimized at the L-C circuit's resonant frequency. Using capacitive tuning to minimize the heat generated by recharging is especially important where the target device will be implanted in a body, such as a heart pacemaker or drug pump, because a rise in the implanted device's temperature of even a few degrees Fahrenheit can cause damage to healthy tissue.
The resonant frequency of the target coil circuit will be determined by the coil inductance, the capacitance, and the physical orientation of the various components and their proximity to other conductors. While the inductance of a coil is largely determined by the size of the coil and the number of coil turns used, the inductance of a coil will be decreased as it is brought into proximity with other conductors. An implanted device, such as a heart pacemaker, is typically contained in a conductive case, and therefore its coil has a relatively stable inductance once encased regardless of any other conductors that are moved nearby. Thus, the resonant frequency of the implanted coil circuit will not appreciably change as the external sending coil of the recharging device is brought close by.
Having fixed the target coil's resonant frequency, maximum inductive coupling is achieved by configuring the recharging device's coil to have the same frequency. That is, the recharging device's coil circuit should generate a field having a fundamental frequency equal to the resonant frequency of the target device's coil circuit. The inductance of the sending coil, and therefore the frequency of its generated field, can change as it is moved close to the case of the target device for recharging. This alters the coupling between the sending coil and the target coil, decreasing the energy transfer and resulting in increased heat generation.
The inductance of a coil can also be changed with the passage of time. Even when new, there is typically a slight variation in the inductance from coil to coil due to manufacturing tolerances. Thus, it is not uncommon to find that the coupling between the sending coil and the target coil is not optimal. This wastes energy, increases the recharging time, and can pose a health risk from heating.
From the discussion above, it should be apparent that there is a need for an energy transfer system for inductively transferring power from an external sending coil to an implanted target coil while maintaining resonant coupling between the coils. The present invention satisfies this need.